The attempt of using GaN as a photocathode in SRF Gun II

The photocathodes determine the beam quality in linear accelerators and represent a key component for many accelerator projects. Free-electron lasers (FEL), synchrotron- and THz radiation sources require injector systems with high brightness electron beams.

High quantum efficiency, a long lifetime and good vacuum stability, fast response time and low thermal emittance are desirable parameters for a perfect photocathode used in accelerators. Semiconductors such as GaN and GaAs as novel materials for photocathodes are showing an enormous potential.
GaAs is a well-known material for photocathodes. After activation with caesium and oxygen, it has a high QE for visible light (red or green). An advantage of GaAs is the opportunity of the layers to emit spin-polarized electrons.
GaN is a semi-conductive material and well known for its high QE when lighted with UV light. For improving the QE only caesium for activation is required.
At the moment GaN is used for photocathode-based detectors such as photomultipliers or phototubes and for LEDs. They have characteristics of low dark current, high-speed response and high sensitivity. It is very new for application in SRF Guns. It seems to be more robust and achieves higher QE than other photocathodes [1].
GaN is a semi-conductive material that is well known for its high QE when lighted with UV light. For improving the QE only caesium for activation is required. It has also a wide wavelength range from 100 to 380 nm.

Doping elements for n-type is silicon (Si) and for p-type magnesium (Mg). Mostly p-doped GaN promises better conditions because magnesium atoms increase the minority carrier diffusion length (about 200 nm). MOVPE is the most used technique to produce p-type GaN. Low temperatures are required in comparison to undoped or n-type GaN. Afterwards an annealing process is necessary to remove magnesium-bonded hydrogen. In p-type GaN electron are the minority carriers whereas holes are the majority carriers. The doping is assumed to lower the band bending around the surface. Therefore, the vacuum level is shifted to lower energy than the conductive band minimum in the flat band region.
Activated with a thin alkali metal layer, like caesium, GaN has the ability to lower the surface work function to produce a negative electron affinity (NEA). This effect originates from the surface band bending. Electrons excite over the bandgap and can easily enter into the vacuum.

Crystallinity and surface parameters define the photoemission properties. Modern analytical methods are used for identification of impurities, dislocations and characterization of the crystallinity of the semiconductors and the right cleaning treatment as well as the right caesium rating.
Like caesium telluride cathode it is possible to recover GaN(Cs) about 50% of the original QE with a simple bake out of 200°C and doing a Cs-reactivation to recover the degraded
cathode [2].
A big advantage of visible light cathodes instead of UV cathodes is to relax the drive laser requirements.

[1] Uchiyama, Shoichi et al. 2011. “GaN-Based Photocathodes with Extremely High Quantum Efficiency” 103511(2005):1–4.
[2] Siegmund, O. et al. 2006. “Development of GaN Photocathodes for UV Detectors.” 567:89–92